Method and apparatus for remote talk/listen communication system

ABSTRACT

A vehicle mounted mobile transceiver having a first and second frequency (11) communicates via a secondary mobile unit (51) with a short range portable unit (10). The portable unit (10) can be either a cordless microphone or a transceiver having a third and fourth frequency. The portable unit (10) allows a remotely positioned operator to transmit messages to a base station (13). To ensure security and reliable operation, the portable and vehicle mounted units use range burst signals and missed message signals to monitor receipt of data signals at the portable unit. The portable and secondary mobile unit communicate with one another via a first pair of frequencies and the vehicle mounted mobile transceiver and the base station communicate with one another via a second pair of different frequencies.

This is a continuation-in-part of application Ser. No. 917,923, filed10/10/86, now U.S. Pat. No. 4,748,685.

TECHNICAL FIELD

This invention relates generally to RF communications systems, andparticularly to mobile radio systems. The invention also relates toportable radios that may be used with such a mobile radio system.

BACKGROUND ART

RF communications systems are well known in the art. In many suchsystems, as in public safety applications, vehicles are equipped withmobile transceivers that allow public safety officers to communicatewith one or more base stations and also with other similarly equippedvehicles in the system. With reference to FIG. 1, such a mobile basedsystem typically includes a vehicle (E) mounted transceiver (A),microphone (B), control head (C), and antenna (F). In addition, a basestation (G) communicates with the vehicle mounted equipment andtypically serves as a message dispatch center. As the situation mayrequire, repeaters (not shown) may be provided to extend the effectiverange between the mobile and base station, all as well understood in theart.

The above configuration works well, so long as the public safety officer(or other individual) remains in the vehicle (E). Once the officer movesbeyond the operational range of the microphone (B), however, the officertypically loses the ability to forward communications to the basestation (G). Since the range of the microphone (B) will usually begoverned by the length of the microphone cord (H), and since this cord(H) must usually be relatively short, communications remote from thevehicle (E) are usually rendered difficult if not impossible.

To meet this problem, portable transceivers (I) can be used to allow theofficer to be able to communicate with the base station (G) whenseparated from the vehicle (E). The portable transceiver (I) must, inorder to be effective, emulate the vehicle mounted system in allrelevant aspects, including frequency capabilities, power rating, andsignalling capabilities. In effect, a public safety service thatimplements this system must provide two wholly and independentlyfunctioning radio systems for each of its vehicles. Though thisadequately supports the desired communication function, this solutionalso represents a relatively costly approach.

There therefore exists a need to provide relatively inexpensive,effective, and reliable communications for public safety officers andothers who ordinarily make use of a vehicle mounted transceiver but whomust also carry out operations away from the vehicle from time to time.

SUMMARY OF THE INVENTION

These needs and others are substantially met through provision of theimproved mobile radio communications system disclosed herein. Thissystem makes use of a relatively short range portable transmitter (ortransceiver, as the case may be) that directly communicates with asecondary mobile unit. The secondary mobile unit in turn communicateswith the primary vehicle mounted mobile transceiver. The secondarymobile unit functions to ensure the propriety of communicating with theportable unit, and then acts in concert with the primary mobile unitlike a repeater to allow communication between the portable and the basestation.

The short range needs of the portable unit minimize power requirements,and in part allow the portable unit to be manufactured and operated in arelatively inexpensive manner. In a transmit only configuration, theportable unit essentially operates as a cordless microphone. If desired,a public address system can be provided on the vehicle to allow returnmessages to be audibly broadcast to the portable user. In a transceiverconfiguration, of course, return messages could be returned directly tothe portable unit via a two-way link with the secondary mobile unit.

The portable unit includes a battery to support its portable function.To reduce maintenance and enhance reliability, the battery can beregularly recharged. The secondary mobile unit can have a batterycharging circuit that functions to recharge the portable unit batterywhenever the portable unit is connected thereto.

To ensure that the secondary mobile unit and the portable unit recognizethe legitimacy of each other's communications, subaudible digital codesare transmitted therebetween as necessary. To minimize maintenance,these codes are freshly generated and exchanged each time the portableunit and the secondary mobile unit are physically joined In oneembodiment, these codes are generated by the secondary mobile unit andtransmitted to the portable unit via the battery charger circuit. Inanother embodiment, the battery charger circuit can be controlled toallow a microprocessor in the secondary mobile unit to reset amicroprocessor on board the portable unit When both the portable unitand the secondary mobile unit are transceivers, the possibility existsthat the portable unit may move out of range of the secondary mobileunit, and that an incoming message may be missed or that an outgoingmessage may be prevented. To resolve this, in one embodiment thesecondary mobile unit occassionally broadcasts a range burst signal Ifthe portable unit does not receive such a range burst signal with apredetermined period of time, an out of range alarm can be provided bythe portable unit to the user.

In another embodiment, the portable unit can be configured to provide anacknowledge signal upon receiving a message from the secondary mobileunit. If the secondary mobile unit does not receive such anacknowledgement, it will broadcast a missed message signal. When theportable unit moves back within range, it will receive the missedmessage signal and provide a missed message alert to the user. The usercan then take appropriate action to ascertain the contents of themessage.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other attributes of the invention will become more clear uponmaking a thorough and complete study and review of the followingdescription of the best mode for carrying out the invention,particularly when reviewed in conjunction with the drawings, wherein:

FIG. 1 comprises block diagram depiction of the prior art;

FIG. 2 comprises block diagram depiction of the invention;

FIG. 3 comprises block diagram depiction of the invention;

FIG. 4 comprises a block diagram depiction of the interface between thesecondary mobile unit and the vehicle's communication system;

FIG. 5 comprises a perspective view of the portable unit;

FIG. 6 comprises a bottom plan view of the portable unit;

FIG. 7 comprises a detailed perspective view of the battery charginghub;

FIG. 8 comprises a block diagram depiction of the portable unit;

FIG. 9 comprises a block diagram depiction of a transmitter suitable foruse in the invention;

FIGS. 10a-10c comprise a schematic diagram of the transmitter;

FIG. 11 comprises a schematic depiction of the transmit enable featureof the invention;

FIG. 12 comprises a block diagram depiction of a receiver suitable foruse in the invention;

FIG. 13a and 13b comprise a schematic diagram of the receiver;

FIG. 14 comprises a schematic diagram of interface switches for use inthe secondary mobile unit;

FIG. 15 comprises a schematic diagram of a watch dog circuit;

FIG. 16 comprises a schematic diagram of the battery charging andinterface circuit;

FIG. 17a and 17b comprise a flow chart of the new code generation andtransfer process;

FIG. 18 comprises a flow chart of portable unit operation under certainoperating circumstances;

FIG. 19 comprises a flow chart of secondary mobile unit operation undercertain operating circumstances; and

FIG. 20 comprises a flow chart of portable unit operation under certainoperating circumstances.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, and in particular to FIG. 2, theinvention can be seen to comprise a portable unit transmitter (10) (ortransceiver, as the case may be, and as explained in more detail below)that communicates with a secondary mobile unit (51) that mounts in avehicle (12). The secondary mobile unit (51) connects to a primarymobile unit (11) that in turn communicates with a base station (13). Ineffect, the primary and secondary mobile units (11 and 51) function as arepeater to connect the portable unit (10) to the base station (13).

In an application where the portable unit (10) includes only atransmitter and effectively comprises a cordless microphone, theportable unit 10) must meet certain FCC requirements governing operationof such a device in the United States. For example, the portable unit(10) can only transmit on one of eight frequencies (169.445 MHz, 169.505MHz, 170.245 MHz, 170.305 MHz, 171.045 MHz, 171.105 MHz, 171.845 MHz, or171.905 MHz).

The maximum output power cannot exceed 50 milliwatts, and the maximumemission bandwidth cannot exceed 54 kHz. It should be noted that theseparticular frequencies may give rise to data reception problems, sinceadjacent frequencies can be used for hydrological and meteorologicaldata transmissions as used by the power, petroleum, forest products,business, and railroad radio services. Therefore, care must be taken toensure that the portable unit (10) will not only meet the above notedspecifications, but that the system will also remain relatively immuneto interference problems from adjacent channels.

Referring now to FIG. 3, the secondary mobile unit (51) that interfacesbetween the portable unit (10) and the primary mobile unit (11)essentially comprises a radio receiver (or transceiver when used with aportable transceiver as explained below in more detail). In practice,the secondary mobile unit (51) can be mounted, for example, inside thehousing of another radio interface product such as a Systems 90 module(52) (model number HLN106) as manufactured by Motorola, Inc. The Systems90 module (52) connects between the primary mobile unit microphone (53)and the primary mobile unit (11) via the microphone input port.

The Systems 90 module (52) adds two primary features to a mobile system.First, the Systems 90 module (52) causes the primary mobile unit (11) tobroadcast a subaudible digital signal comprising a unique ID code everytime the operator closes the push-to-talk (PTT) switch on the microphone(53). In this way, a base station (13) equipped to decode the ID codecan identify the mobile it is communicating with, and maintain a recordof such proceedings. Second, the Systems 90 module (52) can detectswitching of an emergency switch (50). When switching is sensed, theSystems 90 module (52) will cause the ID code for the mobile, plus anemergency code, to be broadcast to the base station (13) for at least apredetermined period of time, or until the base station (13)acknowledges receipt of the signal In this way, an operator can pressthe emergency switch (50), and the primary mobile unit (11) willbroadcast this call for help to the base station (13) regardless of whatelse the operator may do. As will be shown below, the secondary mobileunit (51) can interface with such a module (52) and thereby allow theportable unit (10) to have benefit of such features as well.

Referring now to FIG. 4, the secondary mobile unit can be seen asgenerally depicted by the numeral 51 and as configured in conjunctionwith a Systems 90 module (52). The secondary mobile unit (51) includesgenerally an antenna (54) (which may be comprised of the microphone cord(54) for the primary mobile unit (11)), a receiver (120), a secondarymobile unit microprocessor (1701), a battery charging unit (1731), and awatchdog timer (1732).

As will be described below in more detail, the receiver (120) receivesboth voice and data signals from the portable unit (10) and thenfunctions to separate these signals to thereby provide the voice signal(702) directly to the Systems 90 module (52) where it can serve as theaudio input to the primary mobile unit (11). The data signals (1703)from the portable unit (10) are provided to the secondary mobile unitmicroprocessor (1701) (provided, for example, through use of anMC146805F2 as manufactured by Motorola, Inc.).

The microprocessor (1701) functions to decode and act upon the incomingdata signals and will function either to enable a PTT switch in theSystems 90 module (52) by providing an enable PTT signal (1704) or toenable transmission of an emergency signal via the Systems 90 module(52) through provision of an enable emergency signal (1706). Themicroprocessor (1701) can also provide encoded data and reset signals asdescribed below to the battery charging unit (1731) for use in providingnew communication codes to the portable unit (10).

Finally, the watchdog timer (1732) may be utilized to assure regularproper operation of the microprocessor (1701) in accordance with wellunderstood prior art technique.

Referring now to FIG. 5, the portable unit (10) can be housed in a smallplastic housing (21) having a belt clip (22) to allow easy mounting ofthe portable unit (10) on the person of the operator. The portable unit(10) also includes a PTT button (23), an emergency button (24), and amicrophone (26). A loop antenna, such as those used in personal pagers,can be concealed on the inside of the housing (21).

In addition, and with reference to FIG. 6, two battery charging ports(27) can be provided through the housing (21) to allow access to abattery (and other circuitry described below). To complement thecharging ports (27), a battery charging HUB (28) (FIG. 7) can beprovided having a cavity (29) formed therein for receiving the portableunit (10) and having appropriate conductors (31) disposed therein thatmate with the charging ports (27) of the portable unit (10). The batterycharging HUB (28) can be operably configured in conjunction with thesecondary mobile unit (51) as described below in more detail.

Referring now to FIG. 8, the portable unit can be seen as depictedgenerally by the numeral 10. The portable unit (10) includes generally aportable unit microprocessor (801), an input unit (303), a batterycharging unit (302), a transmit enable unit (304), a transmitter (100),a microphone (101), and an antenna (109).

The portable unit microprocessor (801) receives input signals from theinput unit (303) as operated by the operator. The input unit (303)comprises the PTT button (23) and the emergency button (24) referred toabove. The microprocessor (801) responds to these switches by causingthe transmit enable unit (304) to enable the transmitter (100). In theordinary operating mode, the microprocessor can literally be turned offin order to minimize current drain and extend battery life. When theoperator pushes either button, related circuitry activates themicroprocessor and strobes an interrupt to cause the microprocessor todetermine which button was pushed and to react accordingly. If the PTTbutton has been pushed, an appropriate PTT code is transmittedsubaudibly by the portable to the mobile for a predetermined period oftime, such as two seconds. This allows the mobile to identify theportable (this normally requires around 642 milliseconds), and for thebase station to decode and acknowledge the mobile If the PTT buttonremained closed beyond the two seconds, the portable continues to sendthe PTT code. Otherwise, a termination of message signal may be sent.The battery charging unit (302) allows a battery (not shown) to becharged when the portable unit (10) is in the battery charging hub (28)described above The battery charging unit (302) also allows the portableunit microprocessor (801) to be reset by the secondary mobile unit (51)and to allow the secondary mobile unit (51) to transmit data to theportable unit microprocessor (801), as described below in more detail.

In an alternative embodiment, the portable unit (10) can be configuredas a transceiver having a receiver (120) that receives signalstransmitted by the secondary mobile unit (51) and that demodulates thesereceived signals to provide data to the portable unit microprocessor(801) and voice signals to an appropriate audio transducer system (802).

In another embodiment, the portable unit (10) can include additionalbuttons in the input unit (303) as desired to facilitate remote controlof other desired functions, such as a public address system on board thevehicle, flashing lights on the vehicle, sirens on the vehicle, and thelike.

Referring now to FIG. 9, a block diagram depiction of a transmittersuitable for use in the portable unit (10) (or in the secondary mobileunit (51) when the latter is configured as a transceiver) can be seen asgenerally depicted by the numeral 100. The transmitter (100) includesgenerally a microphone (101), a first amplifier (102), alimiter/deviation adjustment unit (103), a summation node (104), afrequency modulatable oscillator (106), a second amplifier (107), afilter (108), and an antenna (109). The first amplifier (102) serves toamplify audio signals provided by the microphone (101), which amplifiedsignals are then passed to the limited/deviation adjustment unit (103)to properly prepare the audio signal for modulation and subsequenttransmission. The processed audio signal is then summed with datasignals from the relevant microprocessor at the summation node (104) andutilized to modulate the oscillator (106). The modulated carrier signalthen passes through the second amplifier (107) and output filter (108)to the antenna (109) that broadcasts the signal.

Referring now to FIGS. 10a-10c, a more detailed description of thetransmitter (100) will be described

The microphone (101) can be provided through use of a crystal microphonesuch as an MK 1301 (when using the transmitter (100) with the secondarymobile unit (51), one would of course substitute an appropriate audiosignal coupling mechanism to allow input of the primary mobile unit (11)audio signal output) The output of the microphone (101) connects to anamplifier stage (102), essentially comprised of a buffer section and anamplification section. The buffer section includes a 9642 transistor(151) having its emitter connected to ground and its collector connectedthrough a 5.1 k ohm resistor (152) to a switched voltage source (153)and also through a 100 k ohm resistor (154) to the base thereof. Thebase of this transistor (151) also connects to ground through a 100 kohm resistor (156), and through a 0.02 microfarad capacitor (157) to theoutput of the microphone (101). In addition, the output of themicrophone (101) connects to a 2.2 k ohm biasing resistor (158) and anappropriate grounded filter capacitor (159).

The collector of the transistor (151) connects through a 2 microfaradcoupling capacitor (161) to a voltage divider comprised of a 47 k ohmresistor (162) and an 11 k ohm resistor (163), and also to the base of a9648 transistor (164). The emitter of this transistor (164) connects toa 91 ohm resistor (166). The collector of this transistor (164) connectsto a 3.3 k ohm biasing resistor (167), and through a 0.1 microfaradcoupling capacitor (168) to the limiter/deviation adjustment stage(103).

The limiter/deviation adjustment stage (103) includes two groundedparallel configured back-to-back diodes (169 and 171) that provide thelimiting function and an adjustable 50 k ohm resistor (172) that, incombination with a 0.1 microfarad capacitor (173), a 0.005 microfaradcapacitor (174), and an appropriate voltage divider comprised of tworesistors (176 and 177), provides the voice deviation adjustmentfunction. The output of this stage (103) connects through an appropriateRF choke (178) to a summation node (179) (FIG. 10b).

The summation node (179) connects to a data signal unit (181) thatcomprises a plurality of capacitors and resistors configured to receiveand shape data signals from the relevant microprocessor (either theportable unit microprocessor (801) or the secondary mobile unitmicroprocessor (701), as the case may be). In particular, the outputfrom the microprocessor passes through a 20 k ohm resistor (182) to agrounded 0.05 microfarad capacitor (183). Next, the signal traverses a36 k ohm resistor (184) and a 0.02 microfarad capacitor (186). Finally,after passing through a 51 k ohm 7.8 k ohm resistor (188) and 500picofarad capacitor (189) to then pass through a varactor (191) to reachthe summation node (179).

The summation node (179) also connects through two series connectedcapacitors (192 and 193) to the base of an M9932 transistor (194) thatcomprises a part of the oscillator (106). The oscillator (106) furtherincludes a resistor (196), that attaches to the emitter of thetransistor (194) and an adjustable inductor (197) and parallel connected57 picofarad capacitor (198) that connect between the collector of thetransistor (194) and the switched power supply (153). In addition, thebase of the transistor (194) connects to a frequency determining network(111) which comprises a series connected crystal (199), a variableinductor (201), and a 36 picofarad capacitor (202) that are connectedbetween ground and the base of the transistor (194).

The collector of the transistor (194) comprises the output of theoscillator (106) and connects through a 2 picofarad coupling capacitor(203) to the amplification stage (107). The amplification stage (107)includes a grounded variable inductor (204) and a 120 picofaradcapacitor (206) coupled by a 91 picofarad capacitor (207). A voltagedivider comprised of a 68 k ohm resistor (208) and a grounded 120 ohmresistor (209) connects to the base of an M9494 transistor (211), theemitter of which connects to a parallel grounded 27 k ohm resistor (212)and a 390 picofarad capacitor (213).

The collector of this transistor (211) connects to a parallel configuredvariable inductor (214) and 9 picofarad capacitor (216), and alsoconnects through a 2 picofarad coupling capacitor (217) to a groundedvariable inductor (218) and a serially connected 13 picofarad capacitor(219) and 70 picofarad capacitor (221). The common node between thelatter two capacitors (219 and 221) connects to a voltage dividercomprised of a 6.8 k ohm resistor (222) and a 6.2 k ohm resistor (223),and also to the base of an M9494 transistor (224), the emitter of whichconnects to a parallel grounded 100 ohm resistor (226) and 390 picofaradcapacitor (227).

The connector of this transistor (224) connects to an inductor (228) andthrough a 16 picofarad coupling capacitor (229) to a grounded 17picofarad capacitor (231) and also to a voltage divider comprised of a6.8 k ohm resistor (232) and a 6.2 k ohm resistor (233). The common nodebetween these resistors (232 and 233) connects to the base of a thirdM9494 transistor (234), the emitter of which connects to a parallelgrounded 510 ohm resistor (235) and 470 picofarad capacitor (237).

The collector of this transistor (234) connects to an inductor (238) andalso to a filter stage (108)(FIG. 10c). The filter stage (108) includestwo grounded capacitors (239 and 241) that are joined by an inductor(242) and through a coupling 22 picofarad capacitor (243) to anappropriate antenna (109).

In a system where the secondary mobile unit (51) and the portable unit(10) both comprise complete transceivers, some means must be providedfor allowing the secondary mobile unit transmitter to have its transmitmode enabled. A voice responsive mechanism could be utilized to enablethe secondary mobile unit transmitter whenever the output of the primarymobile unit (11) indicates the presence of an audible signal Thesemechanisms, however, are typically slow to respond and often result inlost information. Another approach would be to directly access thesquelch detect of the primary mobile unit (11) and connect appropriatewires to it to allow the squelch detect of the primary mobile unit (11)to control the transmit enable function of the secondary mobile unittransmitter. Although this would accomplish the necessary function, thisapproach represents substantial cost from the standpoint of retrofittingsuch a system into an existing mobile radio installation.

In one embodiment of this invention, a transmit enable signal for thesecondary mobile unit transmitter can be provided through appropriatemonitoring of the primary mobile unit's squelch gate status withoutrequiring invasive rewiring of the primary mobile unit (11) itself.Referring to FIG. 11, an embodiment depicting such a squelch gatedetector can be seen as referred to generally by the numeral 700. Thissquelch gate detector (700) functions in conjunction with the primarymobile unit (11) and the control head (701) associated therewith.

The primary mobile unit (11) will typically include a discriminator(702) for receiving a carrier signal as provided thereto and forextracting the information modulated thereon. The discriminator (702)output signal then ordinarily passes through a volume control switch(703) provided in the control head (701) before returning to the primarymobile unit (11) where it usually passes through one or moreamplification and/or filter stages (704) before reaching a squelch gate(706). The squelch gate (706) responds to a squelch detect (707), whichin turn monitors the output of the discriminator (702) to determine thepresence of a viable signal in accordance with well understood prior arttechnique In the absence of a viable signal, the squelch detect (707)will cause the squelch gate (706) to prevent the output of the amplifierstage (704) from reaching the audio power amplifier (708). In thepresence of a viable signal, however, the squelch detect (707) willallow the squelch gate (706) to pass the signal to the audio poweramplifier (708), which in turn amplifies the signal and renders itaudible at the speaker (709).

Pursuant to this embodiment, a switch (711) can be added to allow theinput to the amplifier (704) and squelch gate (706) to be switchedbetween the discriminator (702) output and a predetermined audio signalsource (712). When switched to a first position (indicated in phantomlines by the reference numeral 713), the output of the discriminator(702) will pass through the volume control (703), through the switch(711), and through the amplifier and squelch gate (704 and 706) asdescribed above. When switched to a second position (as indicated inphantom lines by the numeral 714), however, the output of the audiosignal source (712) will instead be provided to the amplifier andsquelch gate (704 and 706).

When switched to the latter position (714), the operation of the squelchgate (706) will continue to remain a function of the output of thediscriminator (702) as monitored by the squelch detect (707). Therefore,the squelch gate (706) will allow the audio signal source signal to passthrough to the audio power amplifier (708) and the speaker (709) whenthe discriminator (702) provides an output indicating the presence ofviable signal In the alternative, the squelch gate (706) will preventthis predetermined audio signal from reaching the audio power amplifier(708) when the discriminator (702) does not output a viable signal asdetected by the squelch detect (707).

The speaker terminals (710) can be monitored to determine the presenceor absence of the audio signal. In particular, each terminal can beconnected through a 0.1 microfarad capacitor (716 and 717) to the inputsof a first operational amplifier (718). In addition, the inverting inputof the operational amplifier (718) can be connected to a 330 k ohmresistor (719) that connects to a positive 5 volt source, and to thenoninverting input thereof through a 10 k ohm resistor (721). Thenoninverting input of the operational amplifier (718) can also beconnected to a grounded 150 k ohm resistor (722). Finally, a 0.001microfarad capacitor (723) can be connected between the two inputs tothe operational amplifier (718), and a second 0.001 microfarad capacitor(724) can be connected between the noninverting input and ground.

The output of this operational amplifier (718) connects through a diode(726) to the noninverting input of a second operational amplifier (727),which input also connects to a grounded parallel configured 82 k ohmresistor (728) and 0.15 microfarad capacitor (729). The inverting inputof this second operational amplifier (727) connects to a referencevoltage (V_(REF)) having a value, for instance, of 1.8 volts.

So configured, the output (731) of the second operational amplifier(727) comprises a transmit enable port that can be utilized by thesecondary mobile unit transmitter to enable transmission of the audiosignal then being provided at an appropriate port (732) at the output ofthe discriminator.

Through provision of the above described embodiment, easily availableconnections to the existing radio equipment can be made whilesimultaneously assuring reliable and effective squelch detect andtransmission enabling.

Referring now to FIG. 12, a block diagram depiction of a receiversuitable for use in the secondary mobile unit (51)(or in a transceiverconfigured portable unit (10)) can be seen as generally depicted by thenumeral 120. The receiver (120) includes generally an antenna (121), apreselector (122), a first mixer (123) for mixing the output of thepreselector (122) with the output of a first local oscillator (124), anIF section (126), a second mixer (127) for mixing the output of the IFsection (126) with the output of a second local oscillator (128), afilter/limiter/discriminator (129) for extracting the information signalcontained in the incoming signal, an amplifier output stage (131), avoice processing channel (132), and a data processing channel (133).

In the secondary mobile unit (51) the output (134) of the voiceprocessing channel (132) can be provided to the audio input port of theprimary mobile unit (11), while in the portable unit (10) this output(134) could be connected to an appropriate audio transducer. The output(136) of the data processing channel (133) can be connected to anappropriate decoding unit, such as a microprocessor, to allow properdecoding of the incoming signal for purposes described below.

Referring now to FIGS. 13a and b, a more detailed description of thereceiver (120) will be provided

The receiver (120) has an antenna (121) that connects to a preselectorcircuit (122) that can be comprised of various capacitors and inductorsas well understood in the art. The preselector (122) functions inaccordance with well understood prior art technique to pass only a rangeof frequencies, which range will contain the carrier frequency ofinterest.

The output of the preselector (122) connects to a first mixer (123).This mixer includes a voltage divider comprised of first and secondresistors (401 and 402) that connects to the base of a 9662 transistor(403). The emitter of this transistor (403) connects through a parallelconnected resistor (404) and 36 picofarad capacitor (406) to the firstlocal oscillator (124) as described below. The collector of thistransistor (403) connects through a 12 microhenry inductor (407) to agrounded 0.001 microfarad capacitor (408) and through a 51 ohm resistor(409) to a battery source (411). The collector of the transistor (403)also connects through a coupling capacitor (20 picofarad) (412) and pasta grounded 22 picofarad filtering capacitor (413) to an IF stage (126).

The first local oscillator (124) referred to above includes a crystal(414) that connects between ground and through a series connectedinductor (416) and 27 picofarad capacitor (417) to the base of a 9932transistor (418). The base of this transistor (418) also connects to avoltage divider comprised of an 82 k ohm resistor (419) abd a 20 k ohmresistor (421), and to a series connected. 150 picofarad capacitor (422)and 91 picofarad capacitor (423). A common node between these twocapacitors (422 and 423) connects to the emitter of the transistor (418)and also to an 820 ohm grounded resistor (424). The collector of thistransistor (418) connects through a variable inductor (426) to agrounded 0.005 microfarad capacitor (427) and to a biased resistor(428). In addition, the collector of this transistor (418) connects to agrounded 57 picofarad capacitor (429) and also through a 1 picofaradcapacitor (431) to a grounded variable inductor (432). The latterinductor (432) connects to a grounded 47 picofarad capacitor (433) andto the first mixer (123) described above.

The IF stage (126) includes a 10.7 MHz ceramic filter (434), the inputof which connects to receive the output of the first mixer (123) and theoutput of which connects to a voltage divider comprises of a 2 k ohmresistor (436) and a 390 ohm resistor (437). The voltage dividerconnects to the base of an M9662 transistor (438), the emitter of whichconnects to a grounded parallel configured 360 ohm resistor (439) and0.01 microfarad capacitor (441). The collector of this transistor (438)connects through a 330 ohm resistor (442) to a biased 51 ohm resistor(443) and to a grounded 0.001 microfarad capacitor (444).

The collector of this transistor (438) connects to the input of a second10.7 MHz ceramic filter (446), the output of which connects to a filtercomprised of a grounded 22 picofarad capacitor (447), a 20 picofaradcoupling capacitor (448), and a grounded 12 microhenry inductor (449).This filter then connects through a 0.001 microfarad coupling capacitor(451) to the input port of an MC3357 low power narrow band FM IFintegrated circuit as manufactured by Motorola, Inc. (see FIG. 13b).This integrated circuit provides oscillator, mixer, limiting amplifier,quadrature discriminator, active filter, squelch, scan control, and muteswitch functions. This integrated circuit (452) provides the secondmixer (127), second local oscillator (128), and discriminator (129)functions. The MC3357 (452) can be configured as depicted with a 10.245MHz crystal (453) and 455 kHz ceramic filters (454 and 456) to providethe above noted functions and to provide at its output a signal thatincludes the audio and subaudible data signalling as received from theportable unit transmitter (100).

This output signal connects through a 1 microfarad capacitor (457) tothe amplifier stage (131). This amplifier includes an LM324 (458), theinverting input of which connects through a series connected 20 k ohmresistor (459) and 5 k ohm resistor (461) to receive the incomingsignal, and with a common node between these two resistors (461 and 459)being connected to a grounded 0.022 microfarad capacitor (462). Theinverting input of the LM324 (458) also connects through a parallelconfigured 330 k ohm resistor (463) and 0.0039 microfarad capacitor(464) to the output thereof. The noninverting input of the LM324 (458)connects to a grounded 10 microfarad capacitor (466) and to a voltagedivider comprised of a 36 k ohm resistor (467) and a 18 k ohm resistor(468) that is biased between ground and a positive 5 volt source. Soconfigured, this section provides both amplification and deemphasis.

The output of this amplifier stage (131) branches into both a voiceprocessing path (132) and a data processing path (133). The voiceprocessing path (132) essentially comprises a high pass filter followedby a buffer circuit. More particularly, the input to the voiceprocessing path (132) connects through a 0.022 microfarad capacitor(469), a grounded 18 k ohm resistor (471), a 0.022 microfarad capacitor(472), and a 0.022 microfarad capacitor (473) to connect to thenoninverting input of an LM324 (474). The noninverting input of theLM324 (474) also connects to a grounded 120 k ohm resistor (476) andresponds operably to the collector of a 9642 transistor (477), theemitter of which connects to ground and the base of which connects to a5 k ohm resistor (478) to receive a squelch signal from the secondarymobile unit microprocessor (701).

The collector of this transistor (477) also connects through a 6.8 k ohmresistor (479) to the output of the LM324 (474), which also directlyfeeds back to the inverting input thereof The output of the LM324 (474)connects through a 10 microfarad capacitor (481) to a voltage dividercomprised of a, 4.7 k ohm resistor (482) and a 13 k ohm resistor (483)that is biased between a positive 5 volt source and ground. The voltagedivider in turn connects to the base of a 9642 transistor (484), theemitter of which connects to a grounded 1.5 k ohm resistor (486) and thecollector of which connects to the microphone high input line of theprimary mobile unit (51) in the vehicle.

The data processing path (133) receives its input through a 360 k ohmresistor (487), which in turn connects through a 120 k ohm resistor(488) to the noninverting input of an LM324 (489) and through a 0.012microfarad capacitor (491) to the inverting input thereof. Thenoninverting input also connects to a grounded 0.0018 microfaradcapacitor (492), and the inverting input connects to the output thereof.In addition, the output connects through a 30 k ohm resistor (493) tothe noninverting input of a second LM324 (494), with the noninvertinginput also connecting through parallel connected back-to-back diodes(496 and 497) to the inverting input thereof, and to a series connected100 k ohm resistor (498) and 0.68 microfarad capacitor (499). The outputof the second LM324 (494) comprises the data output port (136) that maybe appropriately connected to a relevant microprocessor (either in thesecondary mobile unit (51) or the portable unit (10)) to allow decodingof the data signals. So configured, the second LM324 (494) in the dataprocessing channel (133) serves as a limiter, and the first LM324 (489)serves as a low pass filter.

Referring now to FIG. 14, the secondary mobile unit microprocessor (801)can have one output connected to a switch (1601) comprised of a 10 k ohmresistor (601) and a 9642 transistor (602). This transistor (602) canhave a grounded emitter and a collector that connects to the emergencyactivation switch as found, for instance, in a Systems 90 module (52).So configured, the secondary mobile unit microprocessor (801) cantrigger the emergency transmission function of the Systems 90 module(52). Another switch (1602) comprised of a similar resistor andtransistor can be provided (in addition to a 1 k ohm collector resistor(603)) to allow the microprocessor (801) to control the primary mobileunit's push-to-talk function, and thereby allow a push-to-talk ID to betransmitted by the primary mobile unit (11) and for audio as received bythe secondary mobile unit (51) to be transmitted by the primary mobileunit (11) to the base station (13).

Referring now to FIG. 15, a watch dog timer circuit for use by thesecondary mobile unit (51) can be seen as depicted generally by thenumeral 500. The watch dog timer (500) receives a tickle signal from thesecondary mobile unit microprocessor (801) at an appropriate input(501). This input connects through a 0.1 microfarad capacitor (502) to agrounded 6 k ohm resistor (503) and a series connected diode (504) and 1k ohm resistor (506). This resistor (506) in turn connects to the baseof a 9642 transistor (507) having a grounded emitter. The collector ofthis transistor (507) connects to a voltage divider comprised of two 10k ohm resistors (508 and 509) that are biased by a 5 volt source. This 5volt source also biases a parallel connected 1 microfarad capacitor(511) and 51 k ohm resistor (512) that connect to the base of thetransistor (507). The voltage divider connects to the gate of a 9577 PUT(513). The cathode of the PUT (513) connects to ground and the anodeconnects to provide a reset output signal to the secondary mobile unitmicroprocessor (801). In addition, this output (514) connects to abiased 51 k ohm resistor (516) and a grounded 10 microfarad capacitor(517).

It will be recalled that the portable unit (10) operates with batterypower. For ease of maintenance and to increase reliability, this batterycan be made rechargeable. In particular, a Panasonic P11AA three cellnicad battery pack (which provides a nominal 3.6 volts) can be used.

Referring now to FIG. 16, a battery charging circuit as provided withinthe portable unit (10) can be seen as generally depicted by the numeral300. The portable unit (10) side of the battery charging unit (300)includes generally a battery (301), a data interface unit (302), aninput unit (303), and a transmitter enable unit (304).

The data interface unit (302) includes two input ports (306 and 307)that allow the battery charging unit (300) to be electrically connectedto a complementary battery charging system in the secondary mobile unit(51) as described in more detail below. One of these terminals (306)connects to receive a positive voltage and the remaining terminal (307)connects to ground.

The data interface unit (302) further includes a Zener diode (308), 10 kohm resistor (309), and 20 k ohm resistor (311) that connect in seriesbetween the positive terminal (306) and ground. In addition, thepositive terminal (306) connects through a series connected 860 ohmresistor (312) and diode (313) to the positive terminal of the battery(301). The common node between the 10 k ohm resistor (309) and the 20 kohm resistor (311) connects to the base of a 9642 transistor (314), theemitter of which connects to ground and the collector of which comprisesa reset port that can be connected to the reset input of the portableunit microprocessor (801). The collector of this transistor (314) alsoconnects to a grounded 10 microfarad capacitor (316) and through a 10 kohm resistor (317) to the cathode side of the previously noted diode(313).

The anode side of this diode (313) connects to a voltage dividercomprised of a 20 k ohm resistor (318) and a 10 k ohm resistor (319).This voltage divider connects to the base of a second 9642 transistor(321) The emitter of this transistor connects to ground and thecollector connects to a 10 k ohm biasing resistor (322) and furtherserves as the data input terminal for the portable unit microprocessor(801).

So configured, the data interface unit (302) serves to provide a resetsignal to the portable unit microprocessor (801) as appropriatelycommanded by the secondary mobile unit (51)(as described below in moredetail) and further serves to detect and provide digitized data to theportable unit microprocessor (801) as modulated with the chargingsignal.

The input unit (303) includes a PTT switch (323) and an emergency switch(324) to allow an operator to awaken and properly control the portableunit microprocessor (801). The PTT switch (323) connects between groundand a parallel connected 0.02 microfarad capacitor (326) and 100 k ohmresistor (327), which in turn connect in series with a 20 k ohm resistor(328) that connects to the positive side of the battery (301). Theemergency switch (324) is similarly configured with like numeralsreferring to similarly valued and configured components.

So configured, closing either switch (323 or 324) will provide a strobesignal at an interrupt strobe output (329) that is used to activate theportable unit microprocessor (801) in accordance with well understoodprior art technique. In addition, the microprocessor (801) can examineboth a PTT sense input (331) and the emergency sense input (332) toascertain which switch (323 or 324) has been closed and therebyascertain what action is required.

The transmitter enable unit (304) includes a 9643 transistor (333) thathas an emitter connected to the positive terminal of the battery (301)and a collector connected to provide the switched voltage source to thetransmitter (100) described above. The base of this transistor (333)connects through a 5.1 k ohm resistor (334) to the collector of a 9642transistor (336), the emitter of which connects to ground and the baseof which connects through a 20 k ohm resistor (337) to an appropriateoutput enable port of the portable unit microprocessor (801).

So configured, the portable unit microprocessor (801) can enable thetransmitter (100) by allowing the battery (301) to provide power to thetransmitter (100) components.

With continued reference to FIG. 16, the secondary mobile unit batterycharging circuit can be seen as generally depicted by the numeral 350.This battery charging circuit (350) includes a connection to the vehiclebattery (351) that connects through a 920 ohm resistor (352) to aportable unit sense unit (353), a reset command unit (354), and a datamodulation unit (356).

The portable unit sense unit (353) includes a series connected 6.8 voltZener diode (357), a 10 k ohm resistor (358), and a 4.7 volt Zener diode(359). The output (361) of the portable unit sense unit (353) provides asignal to the secondary mobile unit microprocessor (701) indicating whenthe portable unit (10) has been connected to the battery chargingterminals (362 and 363) of the secondary mobile unit charging HUB (28)described above.

The reset command u nit (354) includes an input (364) for receiving anappropriate reset signal from the secondary mobile unit microprocessor(701) that connects through a 10 k ohm resistor (366) to the base of a9642 transistor (367). The emitter of this transistor (367) connects toground and the collector connects through a 10 k ohm resistor (368) tothe base of a 9643 transistor (369) and also through a second 10 k ohmresistor (371) to the vehicle's battery, which also connects to theemitter of the 9643 transistor (369). The collector of the lattertransistor (369) connects to the positive battery charger terminal (362)and functions to provide a relatively large signal to the portable unitbattery charger unit (300) that causes the data interface unit (302) toprovide a reset signal at the output port identified therein.

The data modulation unit (356) includes an input port (372) forreceiving data signals from the secondary mobile unit microprocessor(701). This input (372) connects through a 10 k ohm resistor (373) toconnect to the base of a 9642 transistor (374), the emitter of whichconnects to ground and the collector of which connects to the positivebattery charger terminal (362) So configured, data signals as providedby the microprocessor (701) are modulated with the charging signal.These data signals can be interpreted by the data interface unit (302)of the portable unit battery charger unit (300) for subsequent use bythe portable unit microprocessor (801) as described below.

By turning the data transmission transistor (374) in the secondarymobile unit battery charger (350) on and off, the corresponding datasense transistor (321) in the portable unit battery charger circuit(300) will similarly be stitched on and off. In this way, digitized datacan be transferred over the battery charger circuit to the portable unitmicroprocessor (801), where the information can be decoded andappropriately buffered and acted upon. The diode (313) in the portableunit battery charger, of course, functions to protect the battery (301)from discharging when this data transfer occurs.

As a direct result of the above configuration, the two battery chargerconnections between the portable unit (10) and the secondary mobile unitsupport trickle charge for the portable unit battery (301), resetting ofthe portable unit microprocessor (801) on command, and the transfer ofdata between the secondary mobile unit microprocessor (701) and theportable unit microprocessor (801).

To ensure that the secondary mobile unit (51) responds only to theportable unit (10) that is specifically assigned to it, the portableunit (10) and secondary mobile unit (51) use a data transmissioncomprising a plurality of codes, including an ID code, a PTT code, andan emergency code. These codes, which may be subaudibly transmitted, arenewly generated each time the portable unit (10) and secondary mobileunit (51) are physically joined together via the battery charger asdescribed above. To ensure accurate creation, transmission, reception,and usage of these codes, a number of safeguards have been utilized.

With reference to FIG. 17, the code generation process begins with thesecondary mobile unit (51) recognizing that the portable unit (10) hasbeen placed into the battery charging HUB (28)(1001). The mobile unit(51) then generates new codes and transmits them (1002) via modulationof the charging signal to the portable (10). The portable (10) decodesthe signals and buffers the new codes (1003). If the portable (10) isremoved from the charging HUB (28) prior to the time the code transferprocess concludes (1004), both units (10 and 51) will default back tothe original codes. Otherwise, the portable (10) will verify receptionof the new codes by making an appropriate transmission (1007) andcontinuous trickle charging of the portable unit battery (301) willfollow (1008).

As soon as the portable (10) is removed from the battery charging HUB(28)(1009), the portable (10) will send a signal (51) to the mobile toverify the new codes (1011). If the mobile (51) does not receive thecorrect new codes within a predetermined period of time (1012), themobile (51) will sound an audible alert (1013) to let the operator knowthat the portable (10) has not been charged with correct new codes (Theoperator would then reinsert the portable (10) to allow the process torepeat.) If, however, the new codes are received and confirmed by themobile (51)(1014), the mobile (51) sounds an audible signal to confirmreceipt of the correct codes, and the mobile (51) and portable (10) thenfunction with the new codes (1016).

By generating new codes with each connection, only a small likelihoodexists that two different portable units (10) having identical codeswill be used in proximity to one another. And, in the event that such anoccurance happened, the codes could be simply changed by repeating theabove procedure. By use of this system, inventory and service problemsare also minimized, since any portable unit (10) in the fleet can beused with any secondary mobile unit (51) in the fleet by following theabove process.

If desired, only the ID need be renewed with each connection. To ensureyet greater security, however, the PTT code and the emergency codereferred to above can also be generated anew each time the secondarymobile unit (51) also generates a new ID code for the portable unit(10). In this way, each mobile-portable pair will not only have a new,randomly generated ID code each time they are paired, but also new andpresumably unique PTT and emergency codes. This assures greater securityand a smaller likelihood that one mobile-portable pair will interfere inthe operation of other mobile-portable pairs.

During use of the above described system, and with reference to FIG. 18,the portable unit micro-processor (701) will ordinarily be off (1101).If the operator pushes the emergency button (24), the micro-processor(701) activates and determines that the emergency button (24) was thebutton that was closed (1102). lf the button (24) has been pushed apredetermined number of time (such as twice or three times) within fiveconsecutive seconds (1103), the portable unit (10) sends a subaudibleemergency code to the secondary mobile unit (51) for a predeterminedperiod of time (1104), such as ten seconds. This signal can be used asdescribed above by a properly programmed mobile unit, such as a MotorolaMDC 600 equipped unit, to toggle and function in an emergency broadcastformat. This means that the primary mobile unit (11) will send apredetermined emergency code to the base station (13) to alert the basestation (13) to an emergency involving the operator. This signalincludes ID information and the like to allow the base station operatorto ascertain who the emergency involves. (To aid in preventing falsetriggering of the emergency function, the secondary mobile unit (51)could be programmed to require receipt of three consecutive emergencysignal detections from the portable unit (10), which three detectionswould ordinarily require about one and a half seconds before acting asset forth above.)

If the operator pushes the PTT button (23), the microprocessor (701)causes a subaudible PTT code to be transmitted to the secondary mobileunit (51) for at least a predetermined period of time (such as twoseconds)(1106), and also for so long as the PTT button (23) remainspushed (1107). As described above, this PTT code is used by thesecondary mobile unit (51) to enable providing voice transmissions fromthe portable unit (10) to the primary mobile unit (11) for transmissionto the base station (13).

When both the portable unit (10) and the secondary mobile unit (51) areconfigured as transceivers, additional operating concerns becomeapplicable. For instance, due to the limited range of the portable unit(10), the likelihood exists that the portable unit (10) will be carriedout of reception range of the secondary mobile unit (51) from time totime. One embodiment of the invention lets the operator know when theportable unit (10) has moved out of range of the secondary mobile unit(51), and also when the secondary mobile unit (51) has transmitted amessage that the portable unit (10) has not received.

With reference to FIG. 19, the secondary mobile unit microprocessor(701) can be programmed to operate to provide both a range burst and amissed message signal when appropriate. For example, the secondarymobile unit microprocessor (701) can increment a count 1 (901) and thendetermine whether this count 1 exceeds a predetermined threshold(X)(902). If not, the microprocessor (701) can proceed to the nextinquiry. If count 1 does exceed this variable, however, the secondarymobile unit (51) can transmit a range burst signal (903) and then clearcount 1 (904). In this way, the secondary mobile unit (51) will, fromtime to time, transmit a range burst signal to the portable unit (10)which the portable unit (10) can use to confirm within-range status.Typically, the range burst signal can be comprised of a subaudibledigitized code that can be created by the secondary mobile unit (51) andimparted to the portable unit (10) at the same time that the IDs, PTTand emergency codes are created and imparted as described above.

Following the range burst process, the microprocessor (701) determineswhether a message flag has been set (906). This flag may be set by themicroprocessor (701) whenever the microprocessor (701) functions totransmit audio from the primary mobile unit (11) via its own transmitter(100). If the message flag has not been set meaning no message has beensent, the microprocessor (701) can return to other functions (907). Ifthe message flag has been set, however, the microprocessor (701) canincrement a count 2 (908) and then determine whether the portable unit(10) has acknowledged reception of the message (909). This presumes, ofcourse, that the portable unit (10) has been programmed to transmit anacknowledgement code to the secondary mobile unit (51) following receiptof a transmission. If the portable has acknowledged receipt of themessage, the microprocessor (701) will clear the message flag (911),clear count 2 (912), and terminate transmission of any missed messagesignals that may have been previously authorized (913). If, however, theportable unit (10) has not acknowledged receipt of a message (909), themicroprocessor (701) will determine if count 2 exceeds a predeterminedthreshold (Y)(914). If not, the microprocessor (701) will return (907)to other preassigned duties. If count 2 does exceed the predeterminedthreshold (914), however, the microprocessor (701) will transmit amissed message signal (916) and then return (907).

So programmed, the secondary mobile unit microprocessor (701) willsupport both range burst and missed message signal functions. In orderto complement these secondary mobile unit (51) functions, the portableunit microprocessor (801) requires additional programming as well. Withreference to FIG. 20, the portable unit microprocessor (801) can beginby determining whether a range burst transmission from the secondarymobile unit (51) has been detected (921). If not, a count 1 can beincremented (922) and a determination made as to whether this count 1exceeds a predetermined threshold (Z) (923). If count 1 does not exceedthis threshold, the microprocessor (801) can move to an out-of-rangeflag set determination as explained below. If count 1 does exceed thisthreshold (923), however, an out-of-range flag can be set (924) and thesubroutine can continue. If a range burst has been detected (921), theout-of-range flag can be cleared (926) as can count 1 (927).

Following the above determinations and actions, the microprocesor (801)next determines whether the out-of-range flag has been set (928). If ithas been set, the microprocessor (801) provides an appropriate alarm(929) through appropriate sounding of an alarm unit (799) (FIG. 8)provided for that purpose.

Following the above actions, the microprocessor (801) can determinewhether any missed message signals have been received from the secondarymobile unit (51) (931). If not, the microprocessor (801) can return(932). If a missed message signal has been detected (931), however, anappropriate alarm (933) can again be provided to the operator. Theoperator can then take whatever actions are appropriate.

Those skilled in the art will recognize that various modifications andchanges could be made to the above described embodiments withoutdeparting from the spirit and scope of the invention. For example,instead of generating both a new PTT code and a new emergency code, thesecondary mobile unit (51) could generate and transmit to the portableunit (10) a single code to meet the need for a PTT code, and theportable unit (10) could invert this code for use as the emergency code.The secondary mobile unit (51), of course, would be configured torecognize the inverted PTT code as the emergency code, such that itcould respond appropriately. Therefore, it should be understood that theclaims should not be considered as limited to the above describedembodiments, in the absence of express inclusion of such embodiments inthe claims.

I claim:
 1. In a communications system comprising first vehicle mounted transceiver means for receiving an input message and for transmitting an inbound signal comprised of said input message as modulated with a first carrier frequency signal, and for receiving outbound messages as modulated with a second carrier frequency signal, which second frequency is not identical to said first frequency, an improvement comprising:portable transceiver means for transmitting a message signal as modulated with a third carrier frequency signal, and for receiving a message signal as modulated with a fourth carrier frequency signal, which third and fourth frequency are not identical to one another and which are different from said first and second frequencies; and second vehicle mounted transceiver means for receiving message signals from said portable transceiver means and retransmitting them via said first vehicle mounted transceiver means as an inbound signal, and for receiving outbound message via said first vehicle mounted transceiver means and retransmitting them to said portable transceiver means.
 2. The improvement of claim 1 wherein said portable transceiver means provides an acknowledge signal to said second vehicle mounted transceiver means following receipt of one of said outbound messages from said second vehicle mounted transceiver means.
 3. The improvement of claim 2 wherein said second vehicle mounted transceiver means transmits a missed message signal to said portable transceiver means when said second vehicle mounted transceiver means fails to receive said acknowledge signal following transmission of one of said outbound messages to said portable transceiver means.
 4. The improvement of claim 3 wherein said second vehicle mounted transceiver means transmits said missed message signal when said second vehicle mounted transceiver means fails to receive said acknowledge signal within a predetermined time period.
 5. The improvement of claim 3 wherein said portable transceiver includes means for responding to said missed message signal by providing an alert signal.
 6. The improvement of claim 1 wherein said second vehicle mounted transceiver means transmits a range burst signal from time to time.
 7. The improvement of claim 6 wherein said range burst signal is so transmitted periodically within at least a minimum time period between transmissions thereof.
 8. The improvement of claim 7 wherein said portable transceiver means provides an out-of-range signal whenever said portable transceiver means fails to receive a range burst signal within a predetermined period following receipt of a last previous range burst signal.
 9. A method of controlling communications between a portable transceiver and a vehicle mounted transceiver, comprising the steps of:in the vehicle mounted transceiver(a) periodically transmitting a range burst signal; (b) monitoring for receipt of an acknowledge signal following transmission by said vehicle mounted transceiver of a message to said portable transceiver; (c) transmitting a missed message signal when reception of said acknowledge signal fails to occur within at least a predetermined period of time; in the portable transceiver(d) monitoring for receipt of said range burst (e) providing an out-of-range signal when reception of said range burst signal fails to occur within at least a predetermined period of time following said portable transceiver's last reception of a range burst signal; (f) monitoring for receipt of said missed message signal; (g) providing a missed message alert upon receiving said missed message signal. 